The invention relates to a method for establishing a relationship between a stress "sgr" and a strain xcex5 and more particularly to a computer system useful therefor.
A method is disclosed comprising first establishing the relationship between the stress "sgr" and the strain xcex5 is firstly established in step 100 with short-term tests as a function of temperature T. In steps 101 to 104, a Findley model is extended to obtain a relationship between strain xcex5 and stress "sgr" as a function of time t and temperature T. The two models are combined in steps 105 and 106, to obtain a relationship between the stress "sgr" and the strain xcex5 as a function of the time t and the temperature T.
From the prior art, it is known for the relationship between the forces acting in the short term and displacements resulting therefrom to be represented by stress/strain curves. Especially in the field of plastics, such stress/strain curves form an important basis for the design of plastic parts.
The disadvantage with known stress/strain curves is that, because of the large number of tensile tests which are needed, establishing them as a function of temperature is very intensive in terms of time and cost. Furthermore, the known stress/strain curves provide only the relationship between the stress and the strain in short-term tensile tests for the measured temperatures. The profile of the strain as a function of constant loading, depending on its loading time, cannot be established from such stress/strain curves.
The time-dependent inelastic strain component is established in so-called creep tests, in which a constant stress is applied to a sample body for a prolonged time. The disadvantage with this is the periods of time which the creep tests need in order to investigate long-term strain behavior, e.g. after 5, 7, 10 or more years. This data is also a very important basis for the design of plastic parts, but it is often unavailable, or not available with the required accuracy, because of the time taken to carry out the corresponding creep tests.
The object of the invention is therefore to provide an improved method, computer system and computer program product for establishing a relationship between the strain and the stress, time and temperature.
The invention makes it possible to establish a relationship between the stress and the strain as a function of the stress loading time, the level of the stress and also the temperature. The task involved with the tensile tests needed for establishing the corresponding coefficients is in this case minimal.
For example, the extrapolation method according to the invention makes it possible, with a relatively short test time of approximately seven weeks, to obtain information about the material behavior for a loading period of more than one year. This applies both to reinforced thermoplastics and to unreinforced thermoplastics, as well as to other plastic materials.
After required coefficients have been established by means of the extrapolation method according to the invention, the resulting function may be used to describe arbitrary stress/strain curves as a function of the loading time t and the temperature T. Such a function may be implemented in a computer program so that a desired curve profile, or a strain or stress value to be calculated, may be obtained at any time by means of a data processing device, for example a personal computer.
This is advantageous for the design of plastic parts, in particular, and permits configuration of the plastic parts while also taking the stress-strain time/temperature behavior into account, and especially with respect to long-term strain behavior.